The invention relates to an improved hydrocarbon steam cracking method intended to produce light olefin, and more particularly ethylene and propylene.
Steam-cracking made its appearance as early as in 1920 to produce ethylene from ethane, and rapidly became a basic method in petrochemistry, using increasingly heavier charges, ranging up to the processing of vacuum gas oils.
Its principle is based on the instability at high temperature of paraffins and naphthenes compared to that of olefin and aromatics. The principal reactions are the scission of a C-C bond by a homolytic rupture mechanism leading to an olefine and a paraffin, and dehydrogenation. These two reactions are endothermic and therefore promoted by a rise in temperature; they also cause an increase in the number of molecules, which means that they are promoted by low partial pressures of the hydrocarbons to be processed. It is for that reason that this pressure is reduced as much as possible by addition of steam to the reaction medium.
However, it was rapidly noticed that maintaining a hydrocarbon charge at a temperature of 800.degree. C. for a time of the order of a few tenths of seconds led to the rapid formation of coke deposits, which is detrimental under several headings: reduction of heat transfer between the reactor and the charge to be cracked, great rise in the temperature of the reactor skin, with reduction of the useful diameter of the reactor causing an increase in the pressure drop inside the reactor, which leads to the shutdown of the unit for a decoking operation.
The formation of coke is due to secondary reactions such as the formation of condensed polycyclic aromatic hydrocarbons, as well as the polymerization of the olefin formed.
This latter reaction stems from the tendency that olefin have to be polymerized when the temperature is of the order of 500.degree.-600.degree. C.; for that reason, in order to reduce the extent of this secondary reaction, the reaction effluents are cooled rapidly (this is often called "quenching") in order to bring them rapidly from the temperature at which pyrolysis takes place to a temperature of below 500.degree. C., generally by means of an indirect heat exchanger.
It was also noticed that the polymerization of the olefin was promoted by the presence of nickel at the surface of the metal walls of the heat exchanger, which acts as a heterogeneous polymerization catalyst (M. DENTE et al., "Fouling of transferline exchanger in ethylene plants", AICHE Meeting at Houston, Tex., 30 Mar. 1983).
Thermodynamic and kinetic studies of hydrocarbon pyrolysis reactions therefore led, in order to increase the selectivity of the reaction towards the production of olefin to act on the following parameters:
rapid increase of the temperature of the charge to the optimum pyrolysis temperature for a given charge, and maintenance of this temperature as constant as possible in the reaction zone. PA1 reduction of the residence time of the charge in the reaction part. PA1 reduction of the partial pressure of the hydrocarbon charge. PA1 rapid and efficient quenching of the reaction effluents. PA1 (a) pre-heating the charge, diluted with steam, PA1 (b) heating at high temperature of this mixture in pipe-stills in order to limit the residence time of the hydrocarbons during this pyrolysis phase, PA1 (c) rapid quenching of the reaction effluents. PA1 a practically homogeneous wall temperature is obtained in the pyrolysis zone. PA1 it is possible to operate at wall temperatures attaining 1,200.degree. to 1,500.degree. C. exceeding the limits of present metallurgy and enabling the heat flow density and reaction temperature to be increased; this is particularly important in the case of ethane cracking. PA1 increase in the s/v ratio, s being the exchange surface and v the reaction volume; it is thus that in the so-called "millisecond" technique, this ratio is of the order of 120 m.sup.-1, whereas the technique of the invention makes it possible to attain values at least equal to 200 m.sup.-1 and able to attain values of 1000 m.sup.-1. PA1 small pressure drop inside the reaction zone, not exoeeding 0.5 bars. PA1 maximum supply of heat to the reaction zone where the highly endothermic reactions take place, giving a very shallow temperature gradient over the whole of the reaction zone. PA1 inertness of refractory material to the charge and to the pyrolysis products. It has been observed that in the reaction zone the refractory steels containing nickel, when used, PA1 promote the formation of coke. French Patent No. 2 472 035 proposes, to mitigate this drawback, the utilization of hydrocarbon pyrolysis reactors consisting of a refractory steel lined internally with a layer of various inert compounds, among which silicon carbide is cited; but this process is very difficult to use, does not make it possible to increase the wall temperatures of the reactor, and, in addition, the metal-ceramic interface withstands great temperature variations badly. In this present invention, the inertness of the refractory material used in the pyrolysis zone enables coking to be reduced without any drawbacks; it is therefore not necessary to passivate the surface of the reactor, at the beginning and/or during the utilization programme, as is usual when metal reactors are used. PA1 Silicon carbide which is the preferentially used material in the invention, has good mechanical strength, even when raised to a high temperature, which makes it possible to use the reactor-quenching exchanger assembly by using very thin material - of the order of 1 mm for the walls separating the unit channels, this feature promoting all the heat exchanges. PA1 The very good thermal stability of the material used provides important advantages when the plant has to be decoked; thus the decoking can be effected with steam at a very high temperature by continuing to pass a heat exchange fluid through the channels provided for that purpose, or else oxidation can be performed, for example with air, in the presence or not of steam, up to wall temperatures that can attain 1,300.degree. to 1,400.degree. C. PA1 Finally, the material used has a very good erosion resistance, which is an important advantage, especially during decoking periods. PA1 The weight ratio of the dilution steam to the hydrocarbon charge varies according to the charges to be processed. It can be between 0.2 and 1.5; generally, the ratio used is of the order of 0.5 for steam cracking naphtha and of the order of 1 when vacuum gas oil is used. PA1 The heat exchange fluids usable in the pyrolysis zone can be any thermal fluids that are stable at temperatures of the order of 1,200 to 1,500.degree. C.; preference is given to using combustion fumes from burners, or hot recovery gases from other processes. PA1 The fluids usable to cool the effluents in the quenching zone may be, for example, air, alone or mixed with combustion fumes, or else a low temperature and a low pressure steam.
From the technology point of view, these imperatives rapidly led to a general method scheme consisting of:
Technological development has concentrated essentially on the pyrolysis (b) and quenching phase (c) to attempt to meet the imperatives stated above and the diversity of the charges to be processed, which at present range from ethane to vacuum gas oils.
The developement of the steam-cracking pipe-stills was essentially directed towards obtaining shorter residence times and a reduction in the pressure drop which led manufacturers to reduce the length of pipe-stills, and therefore increase the heat flow density.
The increase of this latter factor can be obtained essentially by increasing the skin temperature of the pipe-stills and/or reducing the diameter of the pipes (which enables the s/v ratio to be increased, s being the exchange surface and v the reaction volume).
The progress made in metallurgy in special alloys resisting increasingly high temperatures (INCOLOY 800H, HK40, HP40 for example) have enabled manufacturers of pyrolysis pipe-stills for steam-cracking to increase the operating temperatures of these pipe-stills, the metallurgical limits at present being about 1,100.degree. C.
In addition, technology was developed towards using smaller diameter pipes, placed in parallel in order to maintain a satisfactory capacity and remain within a suitable pressure drop.
Several models of pyrolysis pipe-stills have also been suggested, all aiming to increase the heat flow density towards the beginning of the pyrolysis pipe and reducing it subsequently, either by using pipe-stills of increasing diameter, or assembling at least two pyrolysis pipes into one after certain lengths of the reaction zone (see for example F. Wall et al. Chemical Engineering Progress, December 1983, p. 50-55); noncylindrical pipe-stills have also been described, aiming at increasing the s/v ratio; thus U.S. Pat. No. 3,572,999 uses pipes of oval section and U.S. Pat. No. 3,964,873 claims pyrolysis pipes whose section is of dumb-bell shape.
It is in this way that steam-cracker reactor technology developed, from the utilization of horizontal pipes about 100 m long and internal diameter of about 90 to 140 mm, to the "classic" technology of vertically suspended pipes about 40 m long and diameters of the order of 60 mm operating with residence times of the order of 0.3 to 0.4 s, and finally the so-called "milli-second" technique suggested by PULLMAN-KELLOG (U.S. Pat. No. 3,671,198) which uses vertical and straight pipes about 10 m long, internal diameter 25 to 35 mm, these pipes being raised to temperatures of the order of 1,100.degree. C. (metal utilization limit temperature). The residence time of these charges in this type of still is of the order of 0.07 s; the pressure drop observed is of the order of 0.9 to 1.8 bar and the calculation of the ratio of the exchange surface s to the reaction volume v gives values of the order of 120 m.:
As regards the quenching of the reaction effluent products, technology directs itself towards heat exchangers placed in the pyrolysis reaction effluents transfer lines. Numerous exchangers of this type (often called "TLX", "transfer-line-exchanger") have been described in the prior art, as for example in the U.S. Pat. No. 4,097,544 or in the book of L. Kniel, O. Winter and K. Stork "Ethylene keystone to the petrochemical industry", M. Dekker, New York 1980, p. 137 to 142.
The aim of these exchangers is to obtain a sudden drop of the temperature of the effluent gases of the pyrolysis reactors as rapidly as possible to temperatures at which a secondary reaction of olefin polymerization does not take place.
However, the temperature to which the effluent is lowered at the outlet of the quenching exchanger varies in accordance with the steam-cracked charge. For example, when vacuum gas oils of aromatic character are processed, then among the effluents from steam-cracking is quite a large quantity of condensed polyaromatic fuel oils which cannot be cooled suddenly to a low temperature without producing excessing fouling of the exchanger, likely to limit the operating life of the still. In this particular case, it is generally preferred to carry out the cooling operation in two stages, the first being carried out by indirect quenching in the quenching exchanger to a temperature of the order of 450.degree.-500.degree. C., the second stage consisting of direct cooling by the introduction of cold liquids into the exchanger effluents.
Type "TLX" exchanger manufacturers attempted to reduce the dead volume that exists between the outlets of the pipe-stills and the inlets of the effluents in the quenching exchanger, which generally consists of two or three concentric pipes in order to increase the exchange surface areas. Thus U.S. Pat. No. 4,457,364 describes a TLX type exchanger which includes an effluent gases distributor connector in which a reactor pipe corresponds to two exchanger pipes; however, this connector does not entirely overcome the problems of dead volume between the reactor and the quenching exchanger.
However, none of the technologies described for steam-cracking hydrocarbons was fully satisfactory, in particular, none allowed the reactor temperature to exceed about 1,100.degree. C. and therefore raise the temperatures of the charges excessively rapidly to temperatures at which heat cracking takes place under good conditions. In addition, the maximum heat contribution must be made in the pipe-still zone where the endothermic cracking of the C-C bonds and dehydrogenation take place, which is not achieved satisfactorily in the existing methods.
In addition, the necessity of maintaining a high heat flow led to the reduction of the section of the pyrolysis pipes, which imposes a reduction of their length if it is desired to retain an acceptable pressure drop. In addition, no present methods allow an approximately constant temperature to be obtained all along the reaction zone; moreover, the heterogeneity of the heat flows causes large temperature deviations at the circumference of each pipe.
Finally, systems of TLX type heat exchangers have two major disadvantages: they have a non-negligible dead volume, which is detrimental to rapid quenching, and they are made of refractory steels, containing nickel, which as already stated, is an olefin polymerization catalyst.